Despite improvement in treatment of acute myeloid leukemia (AML), high-risk disease such as complex karyotype AML remains largely refractory to current therapy, and is mostly fatal. Identification of effective therapeutic targets by using candidate gene approaches has been limited by the number and variety of genetic defects associated with AML. To identify new therapeutic targets, I carried out a genome-wide functional screen by using a retroviral library of short hairpin RNAs (shRNAs) in complex karyotype AML cells. I discovered that shRNA mediated depletion of hepatocyte growth factor (HGF), ligand of the receptor tyrosine kinase MET, specifically inhibits growth of AML but not other hematologic cancer cells. MET is a potent oncogene, whose aberrant activation is widely implicated in carcinogenesis, causing enhanced growth, survival, and genomic instability of cancer cells. However, mechanisms of carcinogenic MET signaling are currently not well understood, and HGF/MET signaling is not thought to play a role in AML. To validate this observation in patient specimens, I carried out immunohistochemistry of diagnostic bone marrow biopsies. I observed that HGF is aberrantly expressed and associated with activation of MET in about 15% of patients with AML, including most patients with complex karyotype disease. Analysis of cell lines derived from such patients showed that HGF expression was associated with autocrine activation of its receptor MET. Depletion of HGF or MET using shRNA or inhibition of MET using tyrosine kinase inhibitors and neutralizing anti-HGF antibody profoundly reduced proliferation and induced death of AML cells lines that express HGF but not those that lack HGF expression. This indicates the functional dependence or oncogene addiction to this pathway, and suggests that therapeutic inhibition of HGF/MET signaling may be used to improve the treatment of AML. However, detailed understanding of the molecular mechanisms by which this pathway promotes AML cell growth and survival is currently lacking. By genetically engineering AML cell lines to deplete and express specific signaling molecules, and isolating cell lines that are resistant to HGF/MET inhibition, I will identify signaling components that mediate HGF/MET oncogene addiction, and strategies to overcome resistance to therapeutic inhibition of HGF/MET signaling. These studies will be combined with the investigation of antileukemic efficacy of HGF/MET inhibition in murine models of AML in vivo using functional nanoimmunoassay, phosphoproteomic and genomic methods to identify not only the optimal strategy to target this pathway clinically, but also how to optimally integrate it with other targeted inhibitors of AML signaling. These technical advances will circumvent limitations associated with empiric discovery of novel therapeutic targets in AML, and will identify the principal signaling pathways required for AML cell growth and survival. The cell line systems and mouse models will also allow for detailed analysis of gene expression and proteomic changes that accompany signaling by receptor tyrosine kinases, thus providing key mechanistic insights that will be important to a wide variety of biological and disease phenomena. Results of the in vitro and in vivo studies will be further investigated using primary patient specimens, and will lay the foundation for future preclinical studies and clinical trials of targeted therapies of AML. The specific aims are: Specific Aim 1: Dissect the molecular signaling pathways responsible for the HGF/MET dependence of AML cell growth and survival, and identify mechanisms that account for resistance to HGF/MET inhibition (years 1-3). Specific Aim 2: Assess the antileukemic efficacy of HGF/MET inhibition in murine AML models in vivo, both by itself and in concert with the inhibition of other leukemogenic tyrosine kinases, including FLT3 and KIT (years 3-5). The applicant, Dr. Alex Kentsis, a pediatric hematology/oncology fellow at the Dana-Farber Cancer Institute (DFCI) has outlined a 5-year career plan that will build upon his background in biophysics and clinical hematology/oncology. Under the mentorship of Dr. Thomas Look, a recognized leader in cancer cell biology and translational investigations of leukemia, Dr. Kentsis seeks to utilize powerful functional genomic and proteomic approaches using a combination of in vitro systems and murine models in vivo to study the role of aberrant HGF/MET signaling in AML. Dr. Kentsis will be mentored by an Advisory Committee of internationally recognized experts in the field. Finally, the plan is ideally carried out in the Department of Pediatric Oncology at DFCI, given its distinguished record for training physician-scientists in a rich and collaborative environment. With the support provided by the K08 award, Dr. Kentsis' project will lead to the development of clinically effective HGF/MET targeted therapy for AML.